A common therapy for cancerous tumors is radiation treatment, usually by x-rays, gamma rays, or fast electrons, or sometimes by neutrons or heavy charged particles. The success of radiation treatment depends upon the ability of a therapy team to heavily irradiate the tumor volume while delivering a comparatively minimal amount of absorbed dose, or energy imparted per unit mass, to healthy body tissue. To characterize the spatial distribution of dose in a beam of radiation, before applying the radiation dose to the human body, it is common practice to measure the dosage of an impinging radiation beam at a large number of locations throughout a cube-shaped water tank called a "phantom." This is referred to as "mapping the radiation field."
The usual detector employed for taking measurements of the radiation field is a cavity-type ionization chamber attached to a computer-controlled scanning mechanism that is programmed to move throughout the volume of interest. At each point sequentially occupied by the cavity ion chamber, the desired physical quantity to be measured is the absorbed dose in the undisturbed water, i.e., the dose that would be absorbed in the absence of the cavity ion chamber, since the cavity ion chamber perturbs the radiation field. The procedure for interpretation of the electric charge collected in the cavity ion chamber to derive the value of the absorbed dose in water is very complex and error-prone, as described in such documents as "A Protocol for the Determination of Absorbed Dose from High-Energy Photon and Electron Beams," Medical Physics, vol. 10, no. 6, pp. 741-771, November/December 1983. A major disadvantage of the cavity ion chamber is its size (typically 0.1-1.0 cubic centimeters, 1-2 cm in length), which limits the spatial resolution obtainable with such a detector. Smaller cavity ion chambers would suffer from inadequate sensitivity.
Another detector that is sometimes used for mapping the radiation field in the phantom, in place of a cavity ion chamber, is the silicon diode. The silicon diode has the advantages of being smaller than a cavity ion chamber and of being made of a solid instead of a gas, thus avoiding the so-called "displacement or void correction" that must be applied to the cavity ion chamber to account for the lack of beam attenuation in the volume of water displaced by the gas (usually air) in the ion chamber. The offsetting disadvantages of silicon diodes are that they have an atomic number of 14, as compared to one for the hydrogen and 8 for the oxygen in the surrounding water, and their density is 2.3 times that of water. The higher atomic number causes excessive response of the dosimeter to low-energy scattered photons in x-ray or gamma ray beams, and excessive scatter in electron beams. The higher density also perturbs the electrons in the vicinity, thus affecting the dosimeter reading. An example of the use of a silicon diode as a radiation field-mapping probe was published by L. D. Gager et al., "Silicon Diode Detectors Used in Radiological Physics Measurements. Part I: Development of an Energy Compensating Shield," Medical Physics, vol. 4, no. 6, p. 494-498, November/December 1977, and by A. E. Wright et al., "Silicon Diode Detectors Used in Radiological Physics Measurements. Part II: Measurement of Dosimetry Data for High-Energy Photons," Medical Physics, vol. 4, no. 6, pp. 499-502, November/December 1977.